Current control method and circuit for light emitting diodes

ABSTRACT

A flow of a LED current (ILED 2 ) through one or more LEDs is controlled by a LED current control circuit ( 50 - 54 ) implementing a method ( 30 ) that is independent of an input line and a load of the LED(s).

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No.60/468,553, filed May 7, 2003, which the entire subject matter isincorporated herein by reference.

The present invention generally relates to various methods and circuitsfor controlling a light emitting diode (“LED”) current. The presentinvention specifically relates to a regulation of a peak amperage and avalley amperage of a LED current

LEDs are being used more and more in various applications such as, forexample, backlighting, traffic lights, signage, automobiles andillumination. It is well known that a light output of a LED is directlydependent upon a current flowing through the LED. A LED current controlcircuit is therefore used to regulate the current flow through the LEDsto ideally maintain a constant current during all operating conditions.

FIG. 1 illustrates a known LED current control circuit 20 employing aMOSFET switch Q1, a diode D1, an inductor output filter L1, and acapacitive output filter C1 for controlling a flow of a LED currentI_(LED1) through a LED network LED1, LED2. Whenever MOSFET switch Q1 isturned on, circuit 20 controls an increasing flow of LED currentI_(LED1) from a power source (“PS”) 21 through LED network LED1, LED2.Whenever MOSFET switch Q1 is turned off, circuit 20 controls adecreasing flow of LED current I_(LED1) through LED network LED1, LED2.This regulation of LED current I_(LED1) is accomplished by a switchcontrol circuit that employs an operational amplifier U1, a pulse widthmodulation (“PWM”) comparator U2, a gate driver GD1, a sense resistorRs1, and a compensation RC network including an input impedance (“Zi”)22, and a feedback impedance (“Zf”) 23.

In operation, a compensation voltage V_(COMP) generated by inputimpedance 22 and feedback impedance 23 is applied to an inverting inputof operational amplifier U1, and a reference voltage V_(REF1) assupplied by a reference voltage source (“RFS” ) 24 is applied to anon-inverting input of operational amplifier U1. Operational amplifierU1 compares compensation voltage V_(COMP) and reference voltage V_(REF1)to yield an error voltage V_(ERROR), which is an enlarged differencebetween compensation voltage V_(COMP) and reference voltage V_(REF1)that is applied to a non-inverting input of PWM comparator U2. A rampvoltage V_(RAMP) as supplied by a ramp voltage source (“RPS”) 25 isapplied at an inverting input of comparator U2, which compares errorvoltage V_(ERROR) and ramp voltage V_(RAMP) to yield a switching controlvoltage V_(SWC1) for periodically enabling and disabling MOSFET switchQ1 via gate driver GD1.

Ideally, the aforementioned current regulation of LED current I_(LED)maintains LED current I_(LED1) at a mean amperage as illustrated in FIG.2. However, due to an implementation of a 2^(nd) order feedback controlas illustrated in FIG. 1, LED current I_(LED1) can experience anovershoot or an undershoot as illustrated in FIG. 2 whenever a changeoccurs in the input line of power source PS or in a load of the LEDnetwork LED1, LED2. Furthermore, LEDs are frequently non-linear devices,which makes it difficult to design the RC network for optimalperformance.

The present invention addresses the shortcomings with the prior art byproviding a LED current control method and circuit for accurately andquickly regulating the mean amperage of LED current I_(LED) during alloperating conditions including a change in the input line of a powersource or in a change in a load of the LED network.

One form of the present invention is a LED current control method forregulating a peak amperage and a valley amperage of a LED currentflowing through one or more LEDs. First, an upper trip voltage and alower trip voltage are established as control crossover thresholds, anda LED current sensing voltage representative of a flow of the LEDcurrent through the LED(S) is established. Second, a control of anincrease in the LED current from the valley amperage to the peakamperage occurs in response to each crossover of the lower trip voltageby the LED current sensing voltage in a negative direction, and acontrol of a decrease of the LED current from the peak amperage to thevalley amperage occurs in response to each crossover of the upper tripvoltage by the LED current sensing voltage in a positive direction.

A second form of the present invention is a LED current control circuitemploying a hysteretic comparator, a LED current sensor, and aswitch-mode converter for regulating a peak amperage and a valleyamperage of a LED current flowing through one or more LEDs. The LEDcurrent sensor establishes a LED current sensing voltage representativeof a flow of the LED current through the LED(s). The hystereticcomparator establishes an upper trip voltage and a lower trip voltage ascontrol crossover thresholds that are compared to the LED currentsensing voltage. The switch-mode converter controls an increase in theLED current from the valley amperage to the peak amperage in response toeach crossover of the lower trip voltage by the LED current sensingvoltage in a negative direction, and controls a decrease of the LEDcurrent from the peak amperage to the valley amperage in response to

each crossover of the upper trip voltage by the LED current sensingvoltage in a positive direction.

The foregoing forms as well as other forms, features and advantages ofthe present invention will become further apparent from the followingdetailed description of the presently preferred embodiments, read inconjunction with the accompanying drawings. The detailed description anddrawings are merely illustrative of the present invention rather thanlimiting, the scope of the present invention being defined by theappended claims and equivalents thereof.

FIG. 1 illustrates a schematic diagram of a known LED current controlcircuit;

FIG. 2 illustrates a graphical representation of a mean amperage of aLED current controlled by the LED current control circuit illustrated inFIG. 1;

FIG. 3 illustrates a flowchart representative of one embodiment of a LEDcurrent control method in accordance with the present invention;

FIG. 4 illustrates a schematic diagram of one embodiment of a LEDcurrent control circuit in accordance with the present invention;

FIG. 5 illustrates an exemplary graphical representation of a LEDcurrent controlled by the LED current control circuit illustrated inFIG. 4;

FIG. 6 illustrates an exemplary graphical representation of a LEDcurrent sensing voltage generated by the LED current control circuitillustrated in FIG. 4;

FIG. 7 illustrates an exemplary graphical representation of a switchingcontrol voltage generated by the LED current control circuit illustratedin FIG. 4;

FIG. 8 illustrates a schematic diagram of a first embodiment of the LEDcurrent control circuit illustrated in FIG. 4;

FIG. 9 illustrates a schematic diagram of one embodiment of the LEDcurrent control circuit illustrated in FIG. 8;

FIG. 10 illustrates a schematic diagram of a second embodiment of theLED current control circuit illustrated in FIG. 4;

FIG. 11 illustrates a schematic diagram of a third embodiment of the LEDcurrent control circuit illustrated in FIG. 4;

FIG. 12 illustrates a schematic diagram of one embodiment of the LEDcurrent control circuit illustrated in FIG. 11;

FIG. 13 illustrates a schematic diagram of a first embodiment of the LEDcurrent control circuit illustrated in FIG. 12; and

FIG. 14 illustrates a schematic diagram of a second embodiment of theLED current control circuit illustrated in FIG. 12.

A flowchart 30 representative of a LED current control method of thepresent invention is illustrated in FIG. 3, and a LED current controlcircuit 50 for implementing flowchart 30 is illustrated in FIG. 4.During a startup stage S32 of flowchart 30, an upper trip voltage V_(UT)and a lower trip voltage V_(LT) are established as control crossoverthresholds. This is accomplished by an application of a referencevoltage V_(REF2) as supplied by a reference voltage source (“RFS”) 62 toa non-inverting input of hysteretic comparator U3, and an application ofa hysteresis voltage V_(HYS) as supplied by a hysteresis voltage source(“HYS”) 61 to a control input of hysteretic comparator U3. In oneembodiment of hysteretic comparator U3, upper trip voltage V_(UT) equals(V_(REF)+(V_(HYS)/2)) and a lower trip voltage V_(LT) equals(V_(REF)−(V_(HYS)/2)).

In addition, during stage S32, a LED current sensing voltage V_(SEN1) isestablished as a representation of a sensed flow of a LED currentI_(LED2) through LED network LED1, LED2. This is accomplished by a LEDcurrent sensor (“LCS”) 80, which applies LED current sensing voltageV_(SEN1) to an inverting input of hysteretic comparator U3. At a timet₀, LED current I_(LED) is zero (0) amps as illustrated in FIG. 5. LEDcurrent sensing voltage V_(SEN1) consequently is zero (0) volts asillustrated in FIG. 6. Also at time t₀, hysteretic comparator U3 isinitially set to output a switching control voltage V_(SWC2) at a logichigh level LHL as illustrated in FIG. 7 and a power source (“PS”) 60 isapplied to a switch-mode converter (“SMC”) 70 whereby switch-modeconverter is turned on to initiate a flow of LED current I_(LED2) frompower source 60 through LED network LED1, LED2.

Next during startup stage S32, switch-mode converter 70 controls anincrease in the flow of LED current I_(LED2) through LED network LED1,LED2 from zero (0) amps at time t₀ to the peak amperage at time t₁ asillustrated in FIG. 5. LED current sensing voltage V_(SEN1) consequentlyincreases in a positive direction towards upper trip voltage V_(UT) asillustrated in FIG. 6. Upon LED current sensing voltage V_(SEN1)crossing over upper trip voltage V_(UT) during a stage S34 of flowchart30 at a time t₁, hysteretic comparator U3 outputs switching controlvoltage V_(SWC2) at a logic low level LLL as illustrated in FIG. 7whereby switch-mode converter 70 is turned off at time t₁.

During a stage S36 of flowchart 30, switch-mode converter 70 controls adecrease in the flow of LED current I_(LED2) through the LED networkLED1, LED2 from the peak amperage at time t₁ to the valley amperage attime t₂ as illustrated in FIG. 5. LED current sensing voltage V_(SEN1)consequently decreases in a negative direction towards lower tripvoltage V_(LT) as illustrated in FIG. 6. Upon LED current sensingvoltage V_(SEN1) crossing over lower trip voltage V_(LT) during a stageS38 of flowchart 30 at a time t₂, hysteretic comparator U3 outputsswitching control voltage V_(SWC2) at a logic high level LHL asillustrated in FIG. 7 whereby switch-mode converter 70 is turned on attime t₂.

During a stage S40 of flowchart 30, switch-mode converter 70 controls ofan increase in the flow of LED current I_(LED2) through the LED networkLED1, LED2 from the valley amperage at time t₂ to the peak amperage attime t₃ as illustrated in FIG. 5. LED current sensing voltage V_(SEN1)consequently increases in a positive direction towards upper tripvoltage V_(UT) as illustrated in FIG. 6. Upon LED current sensingvoltage V_(SEN1) crossing over upper trip voltage V_(UT) during stageS34 at time t₃, hysteretic comparator U3 outputs switching controlvoltage V_(SWC2) at logic low level LLL as illustrated in FIG. 7 wherebyswitch-mode converter 70 is turned off at time t₃.

As long as power source 60 is being applied to switch-mode converter 70,and reference voltage V_(REF2) and hysteresis voltage V_(HYS) are beingapplied to hysteretic comparator U3, LED current control circuit 50 willcontinue to cycle through stages S34-S40 of flowchart 30 whereby LEDcurrent I_(LED2) continually ramps between the peak amperage and thevalley amperage during a current regulation phase CRP as illustrated inFIG. 5. During the current regulation phase CRP, a mean amperage of LEDcurrent I_(LED2) is an average of the peak amperage and the valleyamperage. As described herein, the peak amperage and the valley amperageare a function of reference voltage V_(REF2) and hysteresis voltageV_(HYS). Consequently, current regulation phase CRP attains a regulationof the mean amperage of LED current I_(LED2) that is accurate during alloperating conditions of circuit 50, including a change in the input lineof power source 60 or a change in the load of the LED network LED1,LED2.

Referring to FIG. 4, circuit 50 can employ any type of switch-modeconverter (e.g., a Buck converter, a Forward converter, a Boostconverter, a Flyback converter, a Bridge converter, and a Cuk converter)to serve as switch-mode converter 70. FIGS. 8-14 illustrate variousembodiments of switch-mode converter 70.

FIG. 8 illustrates a LED current control circuit 51 as an inductiveoutput based version of LED current control circuit 50. To implementflowchart 30 (FIG. 3), circuit 51 employs a switch-mode converter 71including a switching power cell (“SPC”) 90, a gate driver GD2, and aninductive output filter L2. Gate driver GD2 is connected to the outputof hysteretic comparator U3 and a gate terminal G of MOSFET switch Q2whereby MOSFET switch Q2 is opened during stage S36 (FIG. 3) and closedduring stages S32 and S40 (FIG. 3). Inductive output filter L2 isconnected to switching power cell 90 and LED network LED1, LED2 tofacilitate switching power cell 90 in the control of the flow of LEDcurrent I_(LED2) through LED network LED1, LED2.

Switching power cell 90 can include any circuit arrangement encompassingMOSFET switch Q2. FIG. 9 illustrates one such circuit arrangementwherein a drain terminal D of MOSFET switch Q2 is connected to powersource 60, and a source terminal S of MOSFET switch Q2 is connected toinductive output filter L2 and a diode D1, which is also connected to acommon reference CREF. FIG. 9 also illustrates a LED current sensor 81as a version of LED current sensor 80 (FIG. 4). LED current sensor 81includes a resistor R2 having one terminal connected to both LED networkLED1, LED2 and the inverting input of hysteretic comparator U3. Theother terminal of resistor R2 is connected to common reference CREF. Aflow of LED current I_(LED2) through resistor R2 applies LED currentsensing voltage V_(SEN1) to the inverting input of hysteretic comparatorU3.

FIG. 10 illustrates a LED current control circuit 52 as a capacitiveoutput based version of LED current control circuit 50 (FIG. 8). Toimplement flowchart 30 (FIG. 3), circuit 52 employs a switch-modeconverter 72 including switching power cell 90, a capacitive outputfilter C2, gate driver GD2, a logic circuit (“LC”) 100, and a duty cycleoscillator (“DCO”) 110. Logic circuit 100 is connected to hystereticcomparator U3 to thereby receive switching control voltage V_(SWC2).Logic circuit 100 is also connected to duty cycle oscillator 110 tothereby receive oscillating voltage V_(OS). Logic circuit 100 performs alogic operation on switching control voltage V_(SWC2) and oscillatingvoltage V_(OS) to control an opening and closing of MOSFET switch Q2 viagate driver G2 during stage S32, S36 and S40 (FIG. 3). Duty cycleoscillator 110 controls a duty cycle of oscillating voltage V_(OS) tolimit a switching current I_(SW) flowing through MOSFET switch Q2.Capacitive output filter C2 is connected to switching power cell 90 andLED network LED1, LED2 to facilitate switching power cell 90 in thecontrol of the flow of LED current I_(LED2) through LED network LED1,LED2.

FIG. 11 illustrates a LED current control circuit 53 as an alternativeversion of LED current control circuit 52. Circuit 53 employs aswitch-mode converter 73 including a switching power cell 91 havingMOSFET switch Q2 and a switching current sensor (“SCS”) 120 forproviding a switching current sensing voltage V_(SNE2) to a duty cycleoscillator 111. Switching current sensing voltage V_(SNE2) is arepresentation of switching current I_(SW) that enables duty cycleoscillator 111 to output oscillating voltage V_(OS) with a duty cyclethat limits switching current I_(SW).

Referring to FIGS. 10 and 11, the switch-mode converters can employ anytype of logic circuit and duty cycle oscillator. FIG. 12 illustrates aLED current control circuit 54 employing a switch-mode converter 74including a logic circuit in the form of an AND gate 101 and one versionof duty cycle oscillator 111, which employs a comparator U4, a referencevoltage source 112 and a bi-stable device in the form of a RS flip-flop113. Switch current sensor 120 applies switching current sensing voltageV_(SNE2) to an inverting input of a comparator U4. Reference voltagesource 112 applies a reference voltage V_(REF2) to a non-inverting inputof comparator U4, where reference voltage V_(REF2) is a representationof a maximum amperage for switching current I_(SW). An output ofcomparator U4 applies a switching current limit voltage V_(CL) to Rinput terminal of RS flip-flop 113. A clock CLK is applied to S inputterminal of RS flip-flop 113. Q output terminal of RS flip-flop 113 isapplied to a first input of AND gate 101, which receives switchingcontrol voltage V_(SWC2) at a second input. An output of AND gate 101 isconnected to gate driver GD2.

Referring to FIGS. 11 and 12, switching power cell 91 can include anycircuit arrangement encompassing MOSFET switch Q2 and switching currentsensor 120.

FIG. 13 illustrates one circuit arrangement of cell 91 wherein aninductor L3 is connected to power source 60 and a drain terminal D ofMOSFET switch Q2. A diode D2 is connected to drain terminal D of MOSFETswitch Q2. Diode D2 is also connected to capacitive output filter C2 andLED network LED1, LED2. A switching current sensor 121 in the form of aresistor R3 is connected to a source terminal S of MOSFET switch Q2 andcommon reference CREF. The source terminal S of MOSFET switch Q2 is alsoconnected to the inverting input of comparator U4.

FIG. 14 illustrates another circuit arrangement of cell 91 wherein aprimary side of a transformer T1 has one end connected to power source61 and the other end connected to drain terminal D of MOSFET switch Q2.A secondary side of transformer T1 has one end connected to diode D2,and the other end connected to capacitive output filter C2 and senseresistor R2.

For purposes of describing the invention, the LED network LED1, LED2 wasillustrated as a series connection of LEDs. In practice, a LED currentcontrol method and circuit of the present invention can control the LEDcurrent through any number of LEDs having any type of arrangement.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. A method for regulating a mean amperage of a LED current flowingthrough one or more LEDs, the mean amperage being an average of a peakamperage and a valley amperage of the LED current, said methodcomprising: establishing an upper trip voltage and a lower trip voltageas control crossover thresholds; establishing an LED current sensingvoltage representative of a flow of the LED current through the one ormore LEDs; controlling an increase of the LED current from the valleyamperage to the peak amperage in response to each crossover of the lowertrip voltage by the LED current sensing voltage in a negative direction;and controlling a decrease of the LED current from the peak amperage tothe valley amperage in response to each crossover of the upper tripvoltage by the LED current sensing voltage in a positive direction.
 2. ALED current control circuit for regulating a mean amperage of a LEDcurrent flowing through one or more LEDs, the mean amperage being anaverage of a peak amperage and a valley amperage of the LED current,said circuit comprising: a LED current sensor operable to establish anLED current sensing voltage representative of the LED current flowingthrough the one or more LEDs; a hysteretic comparator operable toestablish an upper trip voltage and a lower trip voltage as controlcrossover thresholds, said hysteretic comparator in electricalcommunication with said LED current sensor to receive the LED currentsensing voltage, wherein said hysteretic comparator is operable tooutput a switching control voltage at a first logic level in response toeach crossover of the lower trip voltage by the LED current sensingvoltage in a negative direction, and wherein said hysteretic comparatoris operable to output a switching control voltage at a second logiclevel in response to each crossover of the upper trip voltage by the LEDcurrent sensing voltage in a positive direction; and a switch-modeconverter operable to control a flow of the LED current through the oneor more LEDs, said switch-mode converter in electrical communicationwith said hysteretic comparator to receive the switching control voltage, wherein said switch-mode converter controls an increase of the LEDcurrent from the valley amperage to the peak amperage in response to theswitching control voltage equaling the first logic level, and whereinsaid switch-mode converter controls a decrease of the LED current fromthe peak amperage to the valley amperage in response to the switchingcontrol voltage equaling the second logic level.
 3. The LED currentcontrol circuit of claim 2, wherein said hysteretic comparator includesan inverting input, a non-inverting input, and a control input; whereinthe LED current sensing voltage is applied to the inverting input ofhysteretic comparator; wherein a reference voltage is applied to thenon-inverting input; and wherein a hysteresis voltage is applied to thecontrol input.
 4. The LED current control circuit of claim 2, whereinsaid switch-mode converter includes: a switch in electricalcommunication with said hysteretic comparator to be opened and closed asa function of the switching control voltage.
 5. The LED current controlcircuit of claim 2, wherein said switch-mode converter includes aninductive output filter in electrical communication with the one or moreLEDs.
 6. The LED current control circuit of claim 2, wherein saidswitch-mode converter includes a capacitive output filter in electricalcommunication with the one or more LEDs.
 7. The LED current controlcircuit claim 2, wherein said switch-mode converter includes: a dutycycle oscillator operable to output an oscillating voltage; a logiccircuit in electrical communication with said hysteretic comparator toreceive the switching control voltage and in electrical communicationwith said duty cycle oscillator to receive the oscillating voltage; anda switch in electrical communication with said logic circuit to beopened and closed as a function of the switching control voltage and theoscillating voltage.
 8. The LED current control circuit of claim 7,wherein said switch-mode converter further includes: a switching currentsensor in electrical communication with said switch to output aswitching current sensing voltage representative of a flow of aswitching current through said switch, wherein said duty cycleoscillator is in electrical communication with said switching currentsensor to output the oscillating voltage as a function of the switchingcurrent sensing voltage.
 9. The LED current control circuit of claim 8,wherein said duty cycle oscillator includes: a comparator in electricalcommunication with said switching current sensor to receive theswitching current sensing voltage, said comparator operable to provide aswitching current limit voltage as a function of a comparison of theswitching current sensing voltage to a reference voltage representativeof a maximum amperage of the switching current; and a bi-stable devicein electrical communication to with said comparator to receive theswitching current limit voltage, said bi-stable device operable tooutput the oscillating voltage as a function of a clock signal and theswitching current limit voltage.
 10. A LED current control circuit forregulating a mean amperage of a LED current flowing through one or moreLEDs, the mean amperage being an average of a peak amperage and a valleyamperage of the LED current, said circuit comprising: a LED currentsensor operable to establish an LED current sensing voltagerepresentative of the LED current flowing through the one or more LEDs;a hysteretic comparator operable to establish an upper trip voltage anda lower trip voltage as control crossover thresholds, said hystereticcomparator electrical communication with said LED current sensor toreceive the LED current sensing voltage, wherein said hystereticcomparator operable to output a switching control voltage at a firstlogic level in response to each crossover of the lower trip voltage bythe LED current sensing voltage in a negative direction, and whereinsaid hysteretic comparator is operable to output a switching controlvoltage at a second logic level in response to each crossover of theupper trip voltage by the LED current sensing voltage in a positivedirection; and a switch-mode converter including means for controlling aflow of the LED current through the one or more LEDs as a function ofthe switching control voltage.